In the injection moulding of molten polymers, for example polyethylene, the flow of the polymer into the mould may be an important factor in the manufacture of articles of acceptable properties. Techniques for controlling the flow of molten polymer into a mould are known, for example B. Segmuller, in U.S. Pat. No. 3,533,594 which issued 13th Oct., 1970, describes a hot runner system having branched runner means distributed in at least two planes and in which the lengths of the branched runners between the inlet to the runner system and the moulds are substantially equal. Control of the flow of molten polymer in a runner system attached to a plurality of moulds is important for the uniform filling of the moulds with polymer, uniform filling of the moulds being an important factor in the production of acceptable moulded articles of consistent properties. Similarly, if the runner system is attached to a mould at more than one location, i.e., if the mould has more than one gate, control of the flow of molten polymer in the runner system is important for the uniform filling of the mould with polymer. If the mould is not uniformly filled with polymer the resultant moulded article may be in a stressed condition and the article may warp to reduce the effect of the stresses moulded into the article. For example, in the moulding of an article, e.g. a box, in a mould having four gates located so as to form a square or a rectangle, processing problems may be encountered. Such problems may include entrapment of air, formation of voids and/or creation of regions of stress in the moulded article at points within the location of the four gates.
Injection moulding runner systems capable of providing a uniform flow of molten polymer to a mould, herein referred to as balanced runner systems, and especially runner systems capable of providing a uniform flow of molten polymer to a multi-point gated mould, are therefore desirable. Balanced runner systems and multi-point gating systems are discussed by J. S. Walker and E. R. Martin in "Injection Moulding of Plastics" 2nd Ed., The plastics Institute, Iliffe Books, London, England, especially on pages 99 and 108-109.
A runner system capable of providing a more uniform flow of molten polymer to a mould, and in particular, such a runner system for a five-gated mould, has now been found.